The invention relates to an actuating device for an electrohydraulic vehicle brake system, with an emergency brake cylinder which comprises a piston, the latter being displaceable in a housing, and which can be actuated by means of a brake pedal, and a simulation device for simulating a force/displacement action of the brake pedal, wherein the simulation device is connected in series and disposed after the emergency brake cylinder in the direction of force from the brake pedal to the emergency brake cylinder. The invention also relates to an electrohydraulic vehicle brake system with an actuating device of this kind.
A braking requirement of a driver is detected at the brake pedal, for example, in an electrohydraulic vehicle brake system and, according to this braking requirement and optionally further influencing variables, brake pressure is applied to brakes of the vehicle brake system via an electrohydraulic system (not illustrated in detail here) provided with a motor-driven pressure source. Should the electrohydraulic system fail (for example if the supply voltage in the vehicle drops), this becomes inactive, which may lead to critical running conditions, as no--or only a low--brake pressure can be applied to the brakes by the electrohydraulic system. In order that the vehicle may also be braked in such a case at least with a minimum delay according to legal requirements, a conventional hydraulic emergency actuating system is usually provided in addition to the electrohydraulic system.
DE 196 38 102 C1 describes an actuating device of the type mentioned in which a master brake cylinder can be actuated via a brake pedal. When the electrohydraulic system is inactive the master brake cylinder is connected by valve arrangements to the vehicle brakes in order that the latter may be actuated in an emergency. When the electrohydraulic system is active the master brake cylinder is connected by the valve arrangements to a simulation device which simulates the force/displacement action of the brake pedal which can be experienced by the driver according to a conventional, purely hydraulic vehicle brake system. There is a constant hydraulic connection between pressure chambers of the master brake cylinder and the simulation device when an electrohydraulic system is active. The simulation device is therefore connected in series and disposed after the master brake cylinder in the direction of force from the brake pedal to the master brake cylinder. The simulation device is in this case subject to the high pressure level in the master brake cylinder. This places a substantial load on sealing elements of the simulation device, which reduces their service life and thus the safety of the vehicle brake system.
It is also necessary when actuating the simulation device to overcome the restoring forces of spring elements disposed in the master brake cylinder, which causes difficulty in matching the force/displacement action of the brake pedal which can be experienced, in particular for the initial displacement range, with little force.
A further problem arises if the electrohydraulic system is deactivated or fails while the brake pedal is actuated. In this case the quantity of brake fluid which can be supplied to the vehicle brake is reduced by the quantity which had previously been supplied to the simulation device when the electrohydraulic system was still active. The braking power in an emergency actuation system of this kind is thus restricted and the braking distance of the vehicle undesirably long.